Verneuil crystallizer with powder by-pass means

ABSTRACT

A powder-dispensing assembly for use in combination with a fusion-type crystal-growing furnace in which a carrier gas is used for entraining the powder feed constituents for transportation to the crystal-growing zone thereof. The assembly includes a funnel-shaped powder container positioned an outer container and a means selectively movable for feeding the carrier gas to either the powder container to affect powder fall or to the crystal-growing furnace while simultaneously sealing the powder container to prevent powder fall.

United States Patent [72] Inventors Joseph A. Adamski Farmingham; Walter B. Jackson, Waltham; Robert D. Maher, Dorchester, all of Mass.

[21] Appl. No. 808,124

[22] Filed Mar. 18, 1969 [45] Patented Sept. 21, 1971 [73] Assignee The United States 0! America as represented by the Secretary of the Air Force [54] VERNEUIL CRYSTALLIZER WITH POWDER BY- PASS MEANS 3 Claims, 6 Drawing Figs.

[52] US. Cl 23/273 V, 222/193, 302/52 [51] Int. Cl B01] 17/24 [50] Field of Search 23/273 V,

[56] References Cited UNITED STATES PATENTS 1,653,022 12/ 1927 Schmidt 302/57 2,120,003 6/1938 Schanz 222/193 2,792,287 5/1957 Moore, Jr. et al.... 23/301 SP 2,893,847 7/1959 Schweichert et al. 23/301 SP 3,156,533 10/1964 lmber 23/301 SP 3,185,551 5/1965 Djevahirdjian 23/273 V FOREIGN PATENTS 489,541 12/1929 Germany 302/57 Primary Examiner-Norman Yudkoff Assistant Examiner-R. T. Foster Att0rneys- Harry A. Herbert, Jr. and William J. OBrien ABSTRACT: A powder-dispensing assembly for use in combination with a fusion-type crystal-growing furnace in which a carrier gas is used for entraining the powder feed constituents for transportation to the crystal-growing zone thereof. The assembly includes a funnel-shaped powder container positioned an outer container and a means selectively movable for feeding the carrier gas to either the powder container to affect powder fall or to the crystalgrowing furnace while simultaneously sealing the powder container to prevent powder fall.

PATENTED SEP21 19?: 3.6011 11 SHEET 1 OF 3 TIE 01 TI 6 5 INVENTORS.

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w .4 wZ/ZQW VERNEUIL CRYSTALLIZER WITH POWDER BY-PASS MEANS BACKGROUND OF THE INVENTION This invention relates to an apparatus for growing synthetic crystalline boules. In a more particular manner, this invention concerns itself with an improved fusion-type crystal-growing device which is especially adapted for use with the well-known Vemeuil fusion crystal-growing process.

In the Vemeuil process, crystal growth is achieved by a controlled melting any recrystallization of powdered oxide materials. This process lends itself especially well to the manufacture of synthetic rubies, sapphires, spinels and rutiles.

The Vemeuil apparatus or furnace used in this process is essentially an inverted blowpipe which produces an intensely hot flame by using a mixture of oxygen and hydrogen gases. The crystal-growing powdered oxides are placed in a powder container positioned above the flame. The container is tapped by a suitable mechanism which, in combination with a carrier gas, causes the powder to be carried and passed into the flame. The powder is melted and fused in the intense flame and collects on a crystal-growing surface or support rod where it adheres and slowly forms a pear or carrot-shaped mass, called a boule, with the broader part being uppermost. The support rod is slowly retracted away from the flame so that only the surface of the boule is maintained in the molten state. When a crystal of sufficient size is grown, the furnace is shut down and allowed to cool for a short time. The boule is removed and further cooling takes place rather rapidly. This process produces synthetic gems of good quality and size.

Flame fusion grown crystals, however, often crack when cooled. The resulting fragments, while satisfactory for many purposes, are unsatisfactory when large crystals are required for laser and maser applications. Also, the conventional Verneuil furnace does not permit the addition or interchange of powdered feed constituents without also interrupting the crystal-growing environment. Consequently, crystals of unusual length cannot be grown. Another disadvantage encountered in using the apparatus resides in the fact that it involves a closed system and it becomes necessary to stop the flow of gases while replenishing the powder feed container. As a consequence, single crystals having well-defined areas of differing chemical composition with clearly defined interfaces, such as ruby against sapphire, cannot be grown. Since the length to which the boule can be grown is determined by the size and capacity of the feed feed container, the length of the boule is limited as is its use for laser applications which Oftentimes require crystals of unusual length.

With the present invention, however, the disadvantages encountered when employing a conventional Vemeuil furnace have been overcome by providing a modification which permits a change in the flow path of the carrier gas in such a manner that the gas flow rates remain at the same level throughout the crystal-growing operation. This invention provides a means for controlling the flow of the carrier gas which normally comes from the powder feed container of a conventional Vemeuil apparatus and provides an alternate path for the flow of the carrier to the crystal-growing zone. Thus, the temperature of the growing crystal, the crystal environment, and the liquid-solid crystal-growing interface do not change during the growing operation.

The invention permits the growth of high-quality electronically active single crystals whose chemical composition can be arbitrarily altered without disrupting the actual crystal growth temperature. The concentration and level of impurities can be controlled and changed at any time, for any number of times, in any sequence and for any desired length of crystal. Regardless of the number of changes, the crystal interfaces between any chosen pair of materials remains abruptly sharp. This permits the growth of crystals of any selected composition for any desired length. The temperature of the crystal and crystal-growing environment is never altered while a change in the growth material or an addition of the same growth material is being made.

Previous to this invention, it was not possible to alter the powder feed material initially placed in the container without interrupting the crystal-growing temperature. Previous attempts to solve this problem produced crystals of poor quality, primarily because of uncontrollable or interrupted gas flows. This invention, however, provides a simple and convenient means for adding or substituting pure or impure feed constituents to a Vemeuil crystal-growing apparatus without interrupting the crystal growing environment.

SUMMARY OF THE INVENTION In accordance with the present invention, it has been found that single crystals of high quality, unusual length, and characterized by well-defined areas, each having a different chemical composition can be grown in a Vemeuil fusion apparatus by modifying the oxygen carrier gas inlet means in such a manner that th e carrier gas stream is diverted to either the powder dispensing container or to an alternate path leading to the crystal-growing zone. Normally, the carrier gas enters the powder dispensing assembly through a conduit and entrains the powder constituents. The gas and the entrained powder are subsequently carried to the crystal-growing surface in the growing zone. With this invention, however, the carrier gas enters through a slidable gas carrier tube positioned within a tube support which, in turn. is affixed to the powderdispensing assembly. By properly positioning the slidable tube, the carrier gas flow direction can be changed such that the carrier gas flows to either the powder dispensing assembly for the purpose of entraining the feed constituents or to the crystal-growing zone in order to maintain the intense temperature created by the oxyhydrogen flame. As a consequence, there is no interference with the temperature in the crystalgrowing environment.

Accordingly, the primary object of this invention is to provide an improved apparatus for growing synthetic crystals of gemlike quality.

Another object of this invention is to provide an improved apparatus for growing synthetic crystals having well defined areas with sharply defin ed interfaces in which each area is of a different chemical composition.

Still another object of this invention is to provide an apparatus which permits the growth of crystals of unusual length.

A further object of this invention is to provide an improved flame-fusion crystal-growing apparatus that permits replenishing powdered feed constituents with the same or different crystal fonning powdered constituents without concurrently interferring with the crystal-growing environment.

The above and still further objects and advantages of the present invention will become readily apparent after consideration of the following detailed description thereof when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS in the drawings:

FIG. 1 is a plan view of a flame-fusion crystal-growing apparatus showing in general the modified powder dispensing assembly of this invention;

FIG. 2 is an enlarged plan view, partly in cross section. showing one part of the modified apparatus of FIG. 1;

FIG. 3 is an enlarged view in cross section showing another part of the modified apparatus of FIG. 1;

FIG. 4 is an enlarged isometric view of still another part of the modified apparatus of FIG. I; and

FIG. 5 and 6 are views in cross section, partly fragmented, showing details of a powder dispensing assembly modified in accordance with this invention.

In all the FIGS, the like elements are represented by like numerals.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing FIG. 1 shows in general the details of a Vemeuil crystal-growing apparatus having a carrier gas inlet modified in accordance with this invention. The apparatus described comprises a powder dispensing assembly having an outer housing 112 and a funnel-shaped inner powder feed container 14 mounted within the housing 12. A time wire screen 16 is located in the bottom portion of the container 14. An elongated tube 18 extends downwardly from the housing 12 to a crystal-growing chamber 20. The housing 12 is provided with an inlet 22 for the introduction of a carrier gas to the assembly 10. A cover means 24 for effecting a gastight seal is affixed to the top of the housing 12. A knocker or tapper 26 is suitably connected to an electrical or mechanical means, not shown, for causing the tapper 26 to fall periodically and strike the cover 24.

The crystal-growing zone includes a burner 28 comprising an inner concentric ring formed from the bottom of tube 18 and an outer concentric ring formed from the bottom of outer tube 30 which, in turn, is connected to an inlet 32 for the introduction of hydrogen gas to the burner 28.

Valves 34 and 36 in combination with manometers, not shown, control the flow of calibrated amounts of hydrogen and oxygen gas. The burner 28 opens into the chamber 20 which is preferably formed of fire resistant material, such as fire brick. Within the chamber 20 is positioned a crystal support rod 38, likewise made of fire resistant material, upon which the oxyhydrogen flame from burner 28 impinges. The top of the rod 38 comprises the crystal-growing surface upon which the boule 40 forms when the apparatus is operating. The rod 38, in turn, is supported on a platform 42 which, in turn, is connected to a suitable retraction mechanism 44 for lowering the rod 38 as the boule grows vertically in size.

In this invention, the oxygen carrier gas, which enters through inlet 22, can be diverted through tube 18 and caused to flow directly to the chamber 20 without entering the housing 12 as is done with conventional crystal-growing fusion apparatus. The carrier gas enters through a slidable tube assembly for selectively accomplishing the diversion of the gas flow direction. Specific details of the sliding tube assembly and its operation are shown in FIGS. 5 and 6.

In referring to FIGS 5 and 6, there is shown an outer housing 12, an inner feed container 14 with a wire mesh screen 16 positioned therein. Housing 12 is provided with a gastight cover 24, which, in turn, is sealed against the top of the housing 12 and container 14 by O-ring seals 48 and 50. A suitable solenoid operated tapping mechanism or vibrating means 26 is connected to the feed container 14. With the tapper 26 set in motion, small amounts of the crystal forming feed powder material 46 sifts through the wire screen 16 and falls through the funnel-shaped lower end 52 which, in turn, is connected to the tube 18 for eventual transport of the feed constituents 46 to the crystal-growing zone 20. The housing 12 is provided with an outlet means 21 and a carrier gas inlet 22 for the introduction of oxygen or some other suitable carrier gas. The oxygen, or carrier gas, is introduced through a slidable tube assembly which includes a flexible conduit 53 connected at one end by a nut 54 to a suitable source of oxygen gas, and at the other hand to a slidable gas carrying tube 56 by means of a nut assembly 58. The gas carrying tube 56 is slidably arranged within a gas-carrying tube support 60 which, in turn, is affixed to the housing 12. One end of the support 60 intersects the outlet 21 and gas inlet 22. O-rings 62 and 64, as well as shim spacers 63, 65 and 71 are positioned about the tube 56 to provide a gastight seal between tube 56 and support 60 when backing nut 80 is tightened.

The gas-carrying tube 56 is disclosed in greater detail in FIG. 2 and comprises a hollow central portion 66, and a threaded end 68, for attachment to the nut assembly 58. A latch arm 70 is affixed to the tube 56 and extends outwardly therefrom. The tube 56 is open at ends 72 and 74. An orifice 76 is positioned adjacent to the open end 74.

FIG. 3 discloses details of the gas-carrying tube support 60 and comprises a hollow portion 67 with an inside contour 69 at one end to fit the contour of the gas-carrying tube 56. The support 60 is threaded at one end 78 for attachment to the backing nut 80.

The tube 56, as shown in FIG. 5, is spring biased to the open position by spring 82. A carrier tube holding bracket 84, details of which are shown in FIG. 4, is affixed to the tube support 60 by screws 86. The holding bracket 84 includes shoulder portions 88 and 90 for engaging the latch arm 70 when the gas tube 56 is in the closed position, as shown in F lg. 6. The tube 56 is slidably pushed in and rotated such that the latch arm 70 engages either shoulder 88 or 90 of the bracket 84. The end 74 of the tube 56 seats itself against the inner wall 94 of outlet 21 to seal off the powder container 14 from tube 18. The orifice 76 is positioned in an open manner adjacent to the tube 18. As can be seen by referring to FIG. 6, any oxygen which flows through tube 56 will be diverted to tube 18 for in troduction into the crystal-growing zone 20 without first entering housing 12. Scaling the container 14 prevents the entrainment of any powder particles descending from the feed container 14. Since the feed container 14 is sealed off from the crystal-growing zone, the feed container can now be replenished with the same or different types of powder feed constituents without interfering with the crystal-growing environment.

By sliding the carrier gas tube 56 in and out of the carrier gas tube support 60, the gas stream can be diverted to either the housing 12, or to an alternate path. When the gas-carrying tube is in the closed position, as shown in FIG. 6, the gas flows directly to an alternate path through tube 18 to the crystalgrowing zone 20. At this time the feed container can be replenished with additional powder of the same chemical composition, such as alumina, in order to achieve sapphire crystal growths of unusual length or, alternatively, the feed container may be filled with a powder feed constituent having a different composition, such as chrome-doped alumina, in order to produce a crystal having a combination of materials in a single crystal form, such as ruby and sapphire. The crystals having a combination of materials are marked by well defined interfaces or boundary lines.

When the gas-carrying tube is in the out position, as shown in FIG. 5, the carrier gas fills the entire housing 12 and crystal growth can be resumed in the normal mode. As a result, the gas flow rate and crystal-growing operation. Actual observation of the flame during operation of the apparatus disclosed no evidence of any change in flame composition, temperature or pattern.

The apparatus of this invention is especially well adapted for growing unusually long crystals of one composition, such as sapphire boules, as well as crystals of differing composition, such as alternating areas of sapphire and ruby in the same boule Electron microprope analysis has shown that sapphire on ruby and ruby on sapphire crystals have been grown with well defined boundaries having a boundary thickness of about 20pm. between the red and white portions of the boule. Laue patterns verify the single crystallinity-of the boules. In operating the apparatus of the invention, powdered feed material 46, such as purified alumina, is placed in the feed container 14 on top of the wire screen 16. The cover 24 is tightened to seal the assembly 10. The gas tube 56 is pulled out to the open position and oxygen is admitted to the housing 12 through inlet 22 to entrain falling powder and then through tube 18 to the burner 28. Hydrogen gas is admitted through inlet tube 32 and outer tube 30 to the burner 28. It is to be understood that the gas flow rates will be controlled by reducing valves and manometers, as necessary, and in known manner to produce a flame of desired temperature. The burner 28 is then ignited and the tapper 26 is set in motion. With each blow of the tapper, a small amount of the powdered alumina 46 is sifted through the screen 16 and becomes entrained in the stream of oxygen gas coming into the housing 12 through inlet 22. The carrier gas and entrained powder are carried through the tube 18 into the flame at burner 28 where the powder is fused. The flame and the fused powder impinge upon the top surface of the support rod 38. The fused crystal material accumulates on the surface and forms a molten cap. The rod 38 and support 42 are then slowly lowered by mechanism 44 as the fused material builds up so as to maintain a proper distance between the flame and the growing crystal. As the molten portion of the fused material moves into cooler areas, it crystallizes to form a boule. The quantity and rates of flow for the oxygen and hydrogen are regulated during the formation of the boule to produce a flan having a temperature somewhat higher than the melting point of the feed material 46.

If the replenishment of the feed container is desired at this point, with either the same or a different type of powder feed material, the tapper and retraction mechanism are stopped. This prevents powder drop and maintains the growing boule in a stationary position. The gas flow rates and flame temperature are maintained at the same level as occurred during crystal growth. This is accomplished by simultaneously pushing the tube 56 and latch arm 70 through slot 92 in holding bracket 84 to its closed position. The tube 56 and latch are 70 are rotated to a locking position against either of the holding bracket shoulders 80 or 90. The bottom end 74 of tube 56 seats itself against the inner wall 94 of the lower section of housing 12. This effects a gastight seal between the feed container l4 and the crystal-growing zone 20, thereby allowing the feed material to be replenished or changed as desired. The addition of feed material to the container 14 does not interrupt or interfere with the crystal-growing environment since the carrier gas now passes through tube 56 and orifice 76 directly to tube 18 and burner 28. The crystal-growing temperature and environment are maintained without interruption during the feed-replenishing procedure.

After refilling the feed container, the cover 24 is tightened and the tube 56 is pulled out to its open position. The tapper and retraction mechanism are started and crystal growth resumes in the normal mode. When a boule is allowed to cool. After cooling, the boule is cut from the top surface of rod 38.

The refilling procedure, outlined above, may be repeated any number of times in any given sequence. Consequently, the length of the growing crystal is not limited by the feed container capacity. During the actual operation of the refilling procedure, no perceptible change in crystal growth temperature or alteration in crystal profile temperature was observed. Also, examination of the single crystals obtained after the growing operation revealed no evidence of temperature fluctuations or any sign, such as crystal imperfections, demarcation lines, or undesirable imperfections, that the feed container had been closed off during the refilling operation.

The apparatus can also be used to grow single crystals using feed materials of different composition. A chrome oxide doped aluminum oxide single crystal was grown in the same manner as outlined above. The feed container was shut off from the oxygen carrier gas without interrupting crystal growth temperature and pure aluminum oxide was substituted in the feed container for the chrome-doped aluminum oxide. Single crystal growth was then continued using the apparatus in the normal mode. After a period-of crystal growth, the above procedure was repeated with the exception that chromium doped aluminum oxide was substituted for the pure aluminum oxide. Throughout this procedure, no temperature variations were observed. Extensive visual, optical and microscopic examination of the resulting single crystal revealed no crystal imperfections, such as bubbles, demarcation lines, inclusions or occlusions. Abrupt interfaces indicative of the desired compositional changes, however, were observed.

In an example showing the use of the device of this invention in combination with a three tube burner, a sapphire boule having a diameter of about one inch and a length of about six inches was grown from a powder cone in a period of about twelve hours. Typical conditions during growth was gas flow rates of 6% liters per minute for the inner oxygen, 14 liters per minute for the outer oxygen, and 25 liters per minute for the hydrogen together with a tap rate of 36 taps per minute and a 60-mesh wire screen. The gas flows were regulated at one atmosphere pressure. The powder container was refilled only once with sapphire powder.

In still another example, a single crystal having well defined areas of different chemical composition was grown using powder and a ruby seed crystal to initiate growth. Typical conditions during growth were gas flow rates of from 5 to l4 liters 5 per minute for the outer oxygen, 3 to 6 liters per minute for the inner oxygen, and 25 liters per minute for hydrogen. A tap rate of 34 taps per minute, a SO-mesh screen and a retraction rate of 0.7 inches per hour were also utilized for growing a sapphire area. In initiating growth, the ruby seed crystal was 10 placed on top of the crystal support rod and slowly raised into the flame..The oxygen flow was started with the inner oxygen at about 3 liters per minute and the outer oxygen at about 5 liters per minute until the top of the seed crystal appeared molten. Powder drop was started and when a small crystal began to grow, the retraction rate was set at one inch per hour. Whena rod approximately inches long was grown, the reaction rate was changed to 0.7 inches per hour and the inner and outer oxygen rates increased to widen the crystal. After the crystal had reached a diameter of about 1 inch, the gases were flowing at rates of about 6 liters per minute for the oxygen, 14 liters per minute for the oxygen, while the hydrogen continued at liters per minute.

After minutes of growth, the sapphire powder was taken out of the feed container and exchanged with ruby powder comprising 0.1 percent by weight chrome oxide with the balance pure aluminum oxide. After minutes of growth, the ruby powder was exchanged for pure sapphire. The exchange of ruby powder for sapphire powder, and vice versa, was continued every 40 minutes four more times. After a total growth time of 5% hours, the furnace was shut down and the resultant single crystal was allowed to cool and then removed from the furnace. The crystal had seven alternating areas of ruby and sapphire with well-defined interfaces between each area. 3 The powder dispensing assembly of this invention can be adapted for use with any of the well known fusion type crystal growing furnaces, especially those of the Verneuil type. The stoichiometric composition of the crystalline boules formed by the use of this invention can be easily and closely controlled by adjusting the composition of the feed powder constituents and the amount of powder flow; monitoring the tap rate and retraction rate; providing a calibrated flow control for all gases with the resultant control of flame temperature, pattern and size; and providing precise axial alignment of all components. The effective control of the various growth parameters is achieved without any interference with the crystal-growing environment and results in the growth of gemlike, high-quality electronically active single crystals of unusual length.

The invention has been described with particular reference to a specific embodiment thereof. However, it is to be understood that the description of the invention is for the purpose of illustration only, and it is not intended to limit the invention in any way.

What is claimed is:

1. In a flame-fusion crystal-growing system having a powder dispensing assembly connected with a crystal-growing zone which includes:

an outer container having an outlet in axial alignment with the crystal-growing zone, and

a funnel-shaped powder container positioned within said outer container,

the improvement which comprises means for feeding a powder-entraining carrier gas to said powder dispensing assembly, said means adapted selectively to seal said powder container from the crystal-growing zone while simultaneously diverting the flow of said carrier gas to the crystal-growing zone.

2. The apparatus of claim 1 wherein said means for feeding said carrier gas comprises:

a hollow housing having its outlet end intersecting the outlet of said outer container,

a hollow cylindrical tube positioned within said housing and in slidable contact with the interior portion of said housmg,

tion.

3. The apparatus of claim 2 including means for biasing the cylindrical tube to the open position whereby the carrier gas flows to said powder container for entraining crystal-growing powder feed constituents. 

2. The apparatus of claim 1 wherein said means for feeding said carrier gas comprises: a hollow housing having its outlet end intersecting the outlet of said outer container, a hollow cylindrical tube positioned within said housing and in slidable contact with the interior portion of said housing, said cylindrical tube being movable to a closed position whereby the other end of said tube enters the outlet of said outer container and blocks the outlet to prevent powder fall into the crystal-growing zone while the flow of the carrier gas is simultaneously diverted to the crystal-growing zone and, means for latching the cylindrical tube in closed position.
 3. The apparatus of claim 2 including means for biasing the cylindrical tube to the open position whereby the carrier gas flows to said powder container for entraining crystal-growing powder feed constituents. 